1. Field of the Invention
The invention relates generally to an apparatus used in the field of ophthalmology and more particularly to an incising apparatus used in cataract surgery.
2. Description of the Related Art
The lens of a human eye is a transparent, biconvex crystalline structure located just behind the iris of the eye. The lens substance is contained within an elastic, transparent lens capsule, similar in structure to the substance of a grape being held inside its skin. This lens capsule has a relatively thin cross section, with an average thickness of between 14 microns to 21 microns at its anterior surface.
Cataracts occur when the lens substance opacifies, obscuring the passage of light therethrough and resulting in a decrease in the clarity of a patient's vision. Cataract surgery involves removing the opacified lens and typically replacing it with an artificial intraocular lens implant. The preferred method of routine cataract surgery is referred to as extracapsular cataract extraction. In this procedure the lens is removed through an opening formed in the anterior lens capsule. The remaining portion of the lens capsule is left in place to hold the lens implant that is to be introduced. More modern extracapsular cataract extractions involve the phacoemulsification technique, enabling a smaller surgical incision. The phacoemulsification instrument uses ultrasonic power to fragment the lens nucleus and aspirate the lens contents from the eye. This technique theoretically results in fewer complications, faster healing, and more rapid visual rehabilitation.
A critical and most delicate step in the cataract extraction procedure is the anterior capsulorrhexis step, where a circular opening is created in the thin anterior surface of the lens capsule to obtain access to the lens substance. Prior to starting the capsulorrhexis step, a viscoelastic substance is injected into the eye through a small corneal incision of approximately 1.5 mm (the side port or “paracentesis” port) to maintain proper anterior chamber depth. In current methods, a larger corneal or corneal-scleral incision of 2.0-3.0 mm is then created approximately 3 clock hours from the paracentesis port to accommodate the maneuvering involved in the subsequent capsulorrhexis procedure. A bent sharp-tipped instrument is inserted into the anterior chamber of the eye to make a linear incision in the capsule and to create a small flap. This instrument is then removed, and a forceps is inserted to grasp the newly created free flap. The free flap is delicately manipulated in a circular motion to peel a continuous circular tear in the anterior capsule.
Often, during insertion and manipulation of the forceps, some viscoelastic material inadvertently leaks from the larger corneal incision. This material is critical to maintain the anterior chamber depth and to allow for the successful completion of the circular capsulorrhexis step. Thus, when this material leaks, the forceps must be removed from the eye and more viscoelastic material must be injected. This prolongs the tedious process and increases the risk of error such as a radial tear in the anterior capsule. Because the anterior lens capsule is similar to a cellophane wrapping material, the tear is frequently inadvertently initiated in an undesirable direction, leading to such a dangerous radial tear. A radial tear is dangerous because it results in decreased structural integrity of the lens capsule, and may lead to serious complications, such as the lens falling to the back of the eye or the spillage of vitreous into the anterior chamber intraoperatively. Therefore, it is critical that the tear be continuous and circular, to provide a more stable smooth edge to the opening and to avoid these subsequent radial tears.
The majority of cataract surgeons prefer a round, 5-6.0 mm anterior capsular opening. This size and shape opening allows for better fixation of the nucleus during phacoemulsification and better centration and stability of the intraocular lens implant. Due to the technical difficulty in achieving a capsular opening of optimal size and shape, the anterior capsulorrhexis step may be unreliable, unpredictable, and inconsistent.
U.S. Pat. No. 4,766,897 issued to Smirmaul describes an anterior lens capsule incising apparatus 10. Its forward portion, including a loop 18, can be inserted through a passage wound cut on the corneoscleral tissue of an eye, and disposed above the anterior lens capsule of the eye. A rotatable circular cutting blade 20 of the incising apparatus cuts a circular incision on the anterior lens capsule. The disadvantage of this incising apparatus described in Smirmaul is that it requires a wide passage wound cut on the corneoscleral tissue. The diameter of the rotatable circular cutting blade 20 of Smirmaul is about 6.0 mm, which is the necessary size for cutting an adequate incision on the anterior lens capsule for surgery. Therefore the overall diameter of the circular blade holder must be at least 7.0 mm. This requires that the width of the passage wound cut on the corneoscleral tissue be not less than 7.0 mm, which is wide by eye surgery standards. It is desirable to have the width of the passage wound cut on the corneoscleral tissue as narrow as possible, because a wider wound requires more surgical closing procedures and increases the period of convalescence.
U.S. Pat. No. 5,135,530 issued to Lehmer describes a circular cutting blade having a deformable circular shape with a diameter of not less than 5.0 mm. Additionally, the described preferred anterior lens capsule incising apparatus is able to pass through a narrow corneoscleral tissue wound having a width of not more than 4.0 mm. This deformable circular cutting ring is provided between the forward positions of two elongated arms. The two elongated arms cross each other and are hinged together at the crossing joint. The rearward portion of the two arms are spring biased to keep the forward portion of the two arms spaced apart during use, such that the deformable circular cutting ring maintains its original circular shape. When the rearward portions of the two arms are squeezed toward each other, the forward portions of the two arms similarly move toward each other to compress the deformable circular cutting ring into a narrow elliptical shape. The overall width of the narrow elliptical shaped deformable circular cutting ring and the forward portions of the two elongated arms is less than 4.0 mm. Thus, the narrow elliptical shaped deformed cutting ring and forward portions of the two elongated arms can be inserted into the anterior chamber of the eye through a narrow corneoscleral tissue wound of about 4.0 mm in width. The crossing joint of the two elongated arms is located at or in the vicinity of the corneoscleral tissue wound. Once inside the anterior chamber of the eye, the rearward portions of the two arms are released, so that the forward portions of the two arms move away from each other to allow the deformable circular cutting ring to return to its original circular shape. Then the full size circular cutting ring is pressed onto the anterior lens capsule to cut an adequate sized circular incision so the natural crystalline lens of the eye can be removed and an artificial intraocular lens can be implanted therein. The deformable circular cutting ring is taken out of the anterior chamber through the narrow wound on the corneoscleral tissue by again compressing it into a narrow elliptical shape as noted above.
U.S. Pat. No. 5,728,117, issued to Lash describes a capsulorrhexis instrument that is selectively positionable between a first, retracted position within a tube and a second, extended position projecting out of the tube. The instrument comprises a flexible band having a razor sharp cutting edge which is fixed to a plunger and located within an inserter tube. While in its first, retracted position within the inserter tube, the flexible band assumes an elliptical or oblong shape. However, when the flexible band is moved to its second, extended position outside of the inserter tube such as inside the eye, it deforms into a circular shape whose cutting edge is sufficiently sharp to cut corneal tissue in response to pressure being applied to the corneal tissue by the cutting edge. (While the patent describes corneal tissue being cut, the material to be cut would actually comprise the anterior lens capsule). The dimension to which the flexible band expands to reach the circular shape upon becoming extended from the inserter tube is larger than a cross-section of the gap defined by the inserter tube through which the flexible band travels in its elliptical or oblong shape.
However, none of these apparatus adequately perform their job and are sufficiently complicated and expensive. As a result, there exists a need for a surgical instrument to safely, reliably and consistently create a precise circular anterior capsulorrhexis of proper diameter.